The present invention relates to the application of machine vision algorithms in the manufacture of semiconductor devices. Specifically, the invention applies to the inspection of interconnection lands, or pads, of integrated circuits during fabrication, to ascertain that mechanical probe masks exist, indicating that an electrical test has been performed. The use of an inspection apparatus that can automatically detect and analyze probe masks on device lands can be more reliable and accurate than manual inspection with the human eye.
Silicon device fabrication involves various circuit definition steps with chemical processing of a wafer containing an array of multiple devices. An electrical test of the wafer is typically employed to identify defective devices and/or to perform process control. The electrical test is performed by probing the wafer with needle-tipped probes, leaving an indentation or surface finish aberration on the pad where the probe made contact with the pad. An inspection of the mark left by the electrical testing probe may be performed subsequent to the electrical test to visually assess the electrical testing operations. Absence of a mark, or misalignment of the mark on the pad, would indicate that the test was not properly performed. Additionally, data collected from a probe mark inspection may be useful in process characterization and process control of equipment alignment parameters.
Probe mark inspection is a useful operation in silicon device fabrication because of the increasing size, density, and value of the silicon wafers. Good devices must not be falsely identified as defective, whereas defective devices must be accurately identified so that needles processing and packaging effort is not further explained. Verification of the integrity of the electrical testing operation ensures that the devices are properly tested to avoid the cost penalties of false or improper indictment. Human visual inspection using a microphone is extremely tedious. Thus, the operation is ideally suited for a computer-assisted machine vision application.
Inspection of probe marks on device pads with a machine vision system is difficult because probe marks are relatively unstructured in appearance. A mark indicating acceptance evidence of a probe mark may present many different appearances in an image. Additionally, the pad image may contain features from surface imperfections of the pad that may confuse or obscure the interpretation or presence of a probe mark. Furthermore, particulate deposition on the device pads from the environment may appear similar to the image of a probe mark.
For the foregoing reasons, there is a need for a machine vision apparatus that can inspect silicon device pads on a wafer, and quickly and reliably assess the presence of a probe mark.